Premelting

PVF displays several transitions below the melting temperature. The measured transition temperatures vary with the technique used for measurement. T (L) (lower) occurs at —15 to —20 " C and is ascribed to relaxation free from restraint by crystallites. T (U) (upper) is in the 40 to 50°C range and is associated with amorphous regions under restraint by crystallites (63). Another transition at —80° C has been ascribed to short-chain amorphous relaxation and one at 150°C associated with premelting intracrystalline relaxation. 

Thermal or Flame Spray Process. The earliest experiments in metal spray used molten metal fed to a spray apparatus, where it was dispersed by a high speed air jet into tiny droplets and simultaneously blown onto the surface of the part to be covered. The metal solidified on contact. Modem processes use a more convenient source than premelted metal. Spray heads using a flame or an electrical arc to melt metal wires or powders directly are much more convenient. These are the only types used on a large scale in the United States. 

Recrystallization. The recrystallization of a solid may result in the production of a higher temperature lattice modification, which permits increased freedom of motion of one or more lattice constituents, e.g. a non-spherical component may thereby be allowed to rotate. Such reorganizations are properly regarded as premelting phenomena and have been discussed by Ubbelohde [3]. The mechanisms of phase transitions have been reviewed by Nagel and O Keeffe [21] (see also Hannay [22]). 

These contact angles can be related to the physical state of the surface. The 100 facet is better wetted than the 111 one because the 100 surface is partly premelted. But, the liquid-like disordered monolayer is too thin to have the properties of the macroscopic liquid, and this "adsorbed liquid layer" coexists with a non-wetting macroscopic liquid. This so-called "incomplete surface melting" has also been observed on a pure single crystal of ice. ... 

In both eases, the anisotropy of wetting is driven by the anisotropy of surface energy of the solid. The degree of wetting is shown to be strongly correlated to the degree of premelting at the solid surfaee. 

H. Hakkinen and M. Manninen, Computer Simulation of Disordering and Premelting of Low-Index... 

The same result can be obtained, in a less elegant manner, by a macroscopic heat balance. For the special case of a semi-infinite slab with constant heat input rate H, the temperature distribution in the premelting period has been given previously by Hartree (H6) ... 

During the premelting period the faces are both subjected to boundary conditions of the form... 

The temperature dependence of the chemical shifts of the base and sugar resonances of poly(dA-dT) in 0.1 M phosphate buffer is plotted in Figure 3. There are upfield and downfield shifts associated with the noncooperative premelting transition between 5 and 55°C while only downfield shifts are observed for most of the base and sugar protons on raising the temperature above 65°C in the noncooperative postmelting transition temperature range. 

The duplex to strand transition chemical shift change A6, is defined as the chemical shift difference following extrapolation of the temperature dependent premelting and postmelting shifts to their values at the transition midpoints. The A6 values only approximate the total shift change on proceeding from a stacked duplex to unstacked strands. 

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